1. Field of the Invention
The present invention relates to gate valves, and more particularly to an improved downstream seat and guide structure for gate valves subjected to high pressure differentials across the gate, whereby the force necessary to shift the gate from a closed position to an open position while the valve is subjected to a large pressure differential is substantially reduced.
2. Description of the Related Art
Gate valves for controlling the flow of fluids typically include a slidable gate member that is adapted to be moved from a first position in which a solid portion of the gate blocks the flow through the valve, to a second position in which a flow opening that extends through the gate is aligned with the flow passageway and thus permits flow of fluid through the valve.
The gate is generally a slab-type, planar member having opposed parallel faces that contact and slide across respective upstream and downstream valve seats positioned within the valve body for sealing purposes and through which the fluid flows. The gate includes an offset flow opening of substantially the same diameter as the flow passageway through the valve, the gate opening being adapted to be moved into and out of registry with the flow passageway to alternately permit flow through the valve and to prevent flow therethrough.
Operation of the gate is generally effected by a circular hand wheel that is spaced outwardly from the valve body. The hand wheel is normally positioned at the outer end of a threaded, rotatable stem that extends outwardly from and threadedly engages the valve body. The inner end of the stem is connected with an end of the gate and is freely rotatable relative to the gate so that the linear movement of the stem into and out of the valve body carries with it the gate to control the flow of fluid through the valve.
Positioned against and in facing relationship with each face of the gate is a valve seat, which is generally a ring-shaped structure that has an interior passageway that corresponds in shape and size with the flow passageway through the valve, and which generally includes various types of sealing elements, such as 0-rings, or the like, to effect a seal between the valve seats and the respective faces of the gate, as well as with the valve body within which the seats are carried.
The force necessary to turn the handwheel and shift the gate from a closed, no-flow position to an open, flow position is dependent upon the size of the gate and the contact area between the gate and the valve body, as well as upon the flow area within the valve body and the fluid pressure differential that exists across the valve and the gate. Consequently, for small valves having flow passageways of the order of about 1 to 3 inches or so in diameter and subjected to relatively low pressure differentials, less than about 1,000 psi., normally the valve can be readily opened and closed by manual rotation of a properly sized hand wheel. However, for larger valves, those having flow passageway diameters greater than about 3 inches or so, and also for valves subjected to relatively high pressure differentials, for example 5,000 psi. or greater, oftentimes it is very difficult to manually turn the hand wheel because of the high frictional resistance to gate movement that results from the high upstream pressure exerted against the gate, and that pushes the gate against the interior surfaces of the valve body and downstream seat, with the result that the axial force necessary to move the gate exceeds the force that can be developed by manually turning a hand wheel.
Several approaches have been suggested to reduce the force necessary to operate such high pressure, large diameter, gate-type flow control valves. For example, in U.S. Pat. No. 3,367,625, which issued on Feb. 6, 1968, to Ronald Fortune, there is shown a gate valve that includes a pair of pressurized annular pistons that are aligned with each other and that are each positioned against a respective face of the gate. The annular pistons are of channel-shape in cross section and are slidable axially within annular recesses formed in the valve body and surrounding the flow passageway. The recesses communicate with a source of pressurized fluid that acts to urge each of or only one of the annular pistons against its respective gate face to thereby alter the direction and amount of thrust load acting on either or both sides of the gate.
Another arrangement suggested to reduce the force necessary to move a gate valve gate from a closed position to an open position is shown in U.S. Pat. No. 4,161,309, which issued Jul. 17, 1979, to Thomas A. Klyce. That patent discloses a fluid bypass arrangement wherein high pressure fluid from the high pressure side of the valve is conveyed to and radially expands an elastomeric O-ring into a larger diameter within an annular pocket, to permit flow from the high pressure side of the gate to the low pressure side and thereby equalize the pressures acting on the sides of the gate.
It is an object of the present invention to provide an improved friction-reducing arrangement that avoids subjecting the valve seat sealing surface on the low pressure side of a high pressure gate valve to excessive pressure that could cause premature wear of the sealing surface.